WASHINGTON — President Barack Obama, his voice breaking and tears forming in his eyes, said the killing of 20 children and six adults at a Connecticut elementary school shows the nation must take “meaningful action” to stem such violence.

“There’s not a parent in America who doesn’t feel the same overwhelming grief that I do,” Obama said at the White House, just hours after at least one gunman opened fire at the school in Newtown, Conn. “Our hearts are broken today.”

The president ordered U.S. flags flown at half-staff at all federal buildings and U.S. facilities overseas. He also canceled an appearance next week in Portland, Maine.

Obama focused on the families of the victims and the reaction of the nation, touching only briefing on the subject of ways to address gun violence.

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“As a country, we have been through this too many times. Whether it’s an elementary school in Newtown or a shopping mall in Oregon or a temple in Wisconsin or a movie theater in Aurora or a streetcorner in Chicago, these neighborhoods are our neighborhoods and these children are our children,” Obama said. “And we’re going to have to come together and take meaningful action to prevent more tragedies like this regardless of the politics.”

The Connecticut shooting, the deadliest in a string of mass shootings this year, revived a national debate over gun control laws — a politically fraught issue that the president avoided during his re-election campaign.

White House press secretary Jay Carney said before Obama spoke that reinstating the assault weapons ban “does remain a commitment” for the president. He added that “today is not the day” to revive a policy debate.

New York Mayor Michael Bloomberg, co-chairman of Mayors Against Illegal Guns, said nothing was done after past mass shootings and now there are young schoolchildren dead.

“Calling for ‘meaningful action’ is not enough,” Bloomberg said in a statement. “We need immediate action. We have heard all the rhetoric before. What we have not seen is leadership — not from the White House and not from Congress. That must end today.”

The mayor is the founder and majority owner of Bloomberg News parent Bloomberg LP.

Democratic Rep. Nita Lowey of New York, a longtime advocate of gun-control legislation, said in a statement that “our expressions of sympathy must be matched with concrete actions to stop gun violence.”

Lowey, who has co-sponsored legislation to require gun show operators to register with law enforcement agencies and to keep weapons purchase records, said “we cannot tolerate mass shootings as a mere inconvenience or a normal part of our everyday lives.”

The “easy availability of the deadliest weapons to the most dangerous people has cost countless lives and caused immeasurable suffering, never more so than today,” she said.

Gun control advocates used Twitter to assemble a protest outside the White House to call for stricter gun laws. By the appointed hour, about 100 people had gathered.

“The press secretary said this isn’t the time to talk about it, but it’s the time to talk about it,” Wilson said. “I know we’ve got plenty of other problems, the fiscal cliff and everything, but we can’t forget about this. It’s happening too much.”

Earlier this week, three died after a gunman opened fire inside a mall in suburban Portland, Ore. In August, a shooter took the lives of six people at a Sikh temple in Oak Creek, Wis., before being killed by a police officer. In July, a masked gunman opened fire at a midnight movie in Aurora, Colo., killing 12 and injuring 58.

In the wake of the summer’s shootings and in the midst of his re-election campaign, Obama said he would “examine additional ways that we can reduce violence” even as he resisted calls to push for legislation that would curb such tragedies. Obama and his aides, during the campaign, said the president supported a “common sense” approach to gun control.

During the Oct. 16 presidential debate at Hofstra University in New York, Obama said that in a second term he would look into reinstating the assault weapon ban, something he supported during his 2008 campaign.

Prefacing his answer by saying he believes in the Second Amendment, Obama said he’s committed to enforcing existing laws, making sure they’re keeping guns out of the hands of mentally ill criminals and doing more to enforce background checks. Still, he was noncommittal about making the issue a priority of his second term.

“And so what I’m trying to do is to get a broader conversation about how do we reduce the violence generally,” Obama said at the debate. “Part of it is seeing if we can get an assault weapons ban reintroduced, but part of it is also looking at other sources of the violence, because frankly, in my hometown of Chicago, there’s an awful lot of violence, and they’re not using AK-47s, they’re using cheap handguns.”

With assistance from Roger Runningen, James Rowley and Emma Fidel in Washington.